iwlagn: Enable PCI L1 ACTIVE state after uCode has been loaded

PCI L1 Active needs to be disabled while loading the uCode so that the bus
doesn't go to sleep. The enablement of L1 Active should be done after
the uCode has sent the ALIVE response. The enablement of L1 Active was
missing.
Enabling L1 Active allows to save power if the BIOS / bus driver allows
it.

I measured the power consumption while not associated and
idle/associated:
L1 Active disabled: 39 mA = 130mW
L1 Active enabled:   6 mA = 20 mW

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>